For hydrogeological purposes, the rocks of the Lardarello Basin may be 1 divided into three main complexes:

i) A weakly metamorphic basement complex of quartzites, phyllites, and schists. Although deep exploratory drilling has found occasional fractures and isolated pockets of permeable rock, it is believed that the basement complex is largely impermeable and contributes little to steam production.

ii) A so-called "evaporite" complex of anhydrite, limestones, dolostones, and radiolarites. These rocks are absent in some areas and up to a kilometer thick in others. The limestones and dolostones are known to be highly porous and permeable. The lower-lying anhydrite is believed to be highly porous and permeable where it has been tectonically sheared and brecciated. Because a major regional thrust fault passes through this complex, the tectonically sheared and brecciated zones are believed to be extensive. Overall, this complex is believed to be the main reservoir of liquid water and source of steam in the geothermal system.

iii) A largely sedimentary caprock sequence consisting of unmetamorphosed and weakly metamorphosed shales, marls, feldspathic sandstones, and ophiolitic rocks. Although there are significant volumes of permeable and porous limestones and sandstones in this complex, the preponderance of argillaceous rock types makes it effectively impermeable as a whole. It serves as a caprock for the geothermal system.

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